This invention relates to a method for magnetically recording and reproducing signals in and from a magnetic recording medium via a magnetic head and more particularly, to such a method for magnetically recording and reproducing digital signals in a high density.
Most magnetic recording devices used in computers and video machines rely on the mode of recording and reproducing information signals in and from magnetic recording media by moving electromagnetic induction-type transducer elements as typified by magnetic heads relative to the magnetic recording media in close proximity or contact relationship to the surface thereof. To meet the demands for greater capacity, higher transfer speed and smaller size, the development efforts now made on these magnetic recording devices aim at high density recording. To accommodate such high density recording, magnetic recording media should have magnetic layers which have higher coercivity, reduced thickness and smoothness. On the side of magnetic heads, it is desired to have a narrower gap and higher saturation magnetic flux density.
Magnetic recording media used in the prior art are generally coated magnetic recording media having a magnetic coating layer which is formed by coating a magnetic composition containing magnetic powder and a binder. Such coated magnetic recording media are widely used in the form of magnetic tape and magnetic disks due to their advantage that the magnetic coating has low surface energy and can have a substantial amount of lubricant impregnated in its interior so that it may provide low friction to the head and be well durable against sliding contact with the head. However, the coated media have the drawback that they are low in residual magnetization because the magnetic recording layer contains more amounts of non-magnetic components such as the binder and abrasive and therefore, they cannot be increased in reproduction output. The magnetic recording media of this type use gamma-Fe.sub.2 O.sub.3 and cobalt-adsorbed gamma-Fe.sub.2 O.sub.3 as magnetic powder. These magnetic powders, however, have low coercivity of about 1,000 Oe at maximum and are not sufficient for high-density recording.
Under the circumstances, a switch is partially made to the use of metal thin film type magnetic recording media having a continuous thin film type magnetic layer because they have excellent electromagnetic properties and allow for high-density recording. For the preparation of metal thin film type magnetic recording media, there are known various methods including plating methods such as electroplating, chemical plating, and electroless plating, deposition methods such as vacuum evaporation and sputtering as well as sol-gel methods.
As the magnetic material for the continuous thin film type magnetic disks, metal thin films of CoNiCr, CoNiP or the like and metal oxide thin films of .gamma.--Fe.sub.2 O.sub.3 were developed. Preference is made to metal thin film media capable of reproducing greater outputs by virtue of increased remanence. The metal thin film type magnetic recording media, however, have the drawback that the media are less durable in that their surface is prone to damage by contact with the head. Further the magnetic recording media are less reliable because head adsorption often occurs due to increased friction between the media and the head.
Additionally, the magnetic film is prone to oxidation during long-term storage, leaving problems including occurrence of pinholes in the magnetic film and a lowering of magnetization quantity which leads to output lowering and error occurrence. It is then a common practice to form on the magnetic layer surface a protective layer such as a ceramic film, silicon oxide film and lubricant film, with the results being still unsatisfactory.
To further improve recording density and increase record data transfer speed, it is required to shift the recording frequency to a higher side. Recording at higher frequencies entails the problem that output is dampened by the inductance of the head. This can be overcome by reducing the number of coil turns in the head which in turn, rather causes a lowering of reproduction output. It is then difficult to increase storage capacity.
One solution to these problems is to reduce the inductance of the head. For example, there are known multi-layer heads having alternately deposited films of a high saturation magnetic flux density material and a non-magnetic material on a non-magnetic substrate. Thin film type heads featuring more efficient reproduction are also used to further reduce the inductance.
Also proposed were inductive/magnetoresistance composite thin film heads using a magnetoresistance (MR) element as a reproducing head. Since a significant improvement in reproduction output is achieved by the use of an MR element as the reproducing head, even magnetic recording media having low remanence lend themselves to high density recording/reproducing operation. Use of thin film type magnetic heads including an MR element as the reproducing head and having a low inductance enables high-frequency recording and high-density recording.
Nevertheless, when high-frequency recording is effected on sputtered or plated metal thin film type magnetic recording media using such thin film type magnetic heads, there occurs modulation noise inherent to the metal thin film type media, so that the recording density cannot be increased. More particularly, the metal thin film type magnetic recording media which are prepared by sputtering or plating techniques have uniform, continuous magnetic thin films so that they have high remanence and provide high reproduction outputs. If the recording frequency is increased for high-density recording and high-speed transfer, a drastic increase of modulation noise occurs which prohibits high-density recording. It is contemplated that such modulation noise occurs in the metal thin film type magnetic recording media because in magnetization inverted regions of the magnetic thin film where signals are recorded, the magnetic wall of the inverted boundary moves to provide a heterogeneous boundary so that the magnetization inversion transition region is spread.
One effective approach for minimizing the noise associated with the shifting magnetic wall in metal thin film media is to add non-magnetic material to control the growth of the magnetic wall. This raises the problems of decreased remanence and coercivity squareness ratio. This results in losses of reproduction output and linear recording density, rendering it difficult to improve recording density.
Therefore, the head must be reduced in inductance to increase reproduction output and the recording medium must be reduced in noise before high-frequency recording can be achieved to establish a high-density, high-transfer-speed recording method.
Under the above-mentioned situation, the inventors made research on the magnetic recording of signals in magnetic recording media using magnetic heads so as to ensure high recording density, high-speed data transfer, durability and reliability.
Therefore, an object of the present invention is to provide a magnetic recording/reproducing method featuring high-density recording while ensuring durability and reliability.
Another object of the present invention is to provide a cost-effective magnetic recording/reproducing method using a magnetic recording medium which can be manufactured in high productivity and at low cost.
From this point of view, the inventors previously proposed in JP-A 157801/1991 a method for effecting magnetic recording/reproducing operation on a magnetic disk using a flying magnetic head. The magnetic disk includes a rigid substrate and a magnetic layer formed thereon by coating a magnetic coating composition containing ferromagnetic metal submicron particles. The method is characterized in that the magnetic layer has a coercivity of at least 1,100 Oe and a thickness of up to 0.5 .mu.m, and at least a portion of the flying head that is disposed adjacent a gap is formed of a soft magnetic material having a saturation magnetic flux density of at least 0.7 T.
The magnetic disk of the previous proposal can be manufactured in high productivity and at low cost. The method ensures high-density recording, acceptable overwrite characteristics, and reliable magnetic recording/reproducing operation.
A system wherein the above-mentioned medium is combined with an inductive, MR composite thin film head allows for an increase of recording density because there occurs no noise associated with the magnetic wall which would otherwise be observed in short-wavelength recording of metal thin film media. However, acicular submicron particles, for example, of .alpha.-Fe metal used in the recording medium have magnetic anisotropy which develops due to shape magnetic anisotropy. This implies that variations in shape or particle size are directly reflected by variations in coercivity.
On the other hand, magnetic particles should have a narrow particle size distribution in order to reduce the coercivity distribution which is one of the causes for modulation noise to occur in a coated magnetic film for thereby reducing the noise associated with the medium. However, there is a certain limit in reducing the particle size distribution. Then even when the magnetic recording medium constructed according to the previous proposal is used, we encountered a limit in reducing the noise of the medium. Another problem arose that linear recording density could not be increased since a high coercivity squareness ratio was not available.
To attain the above-mentioned objects, the present invention in a first form provides a method for magnetically recording and reproducing signals in and from a magnetic recording medium via a thin film type magnetic head. The magnetic recording medium includes a non-magnetic substrate and a magnetic layer formed thereon by coating a composition containing hexagonal ferromagnetic submicron particles and a binder. The magnetic layer has a thickness of up to 0.5 .mu.m, a surface roughness (Ra) of up to 5 nm, and a coercivity (Hc) of at least 1,100 Oe and a squareness ratio (S) of at least 0.70 as measured in the direction of movement relative to the head. The magnetic head includes a recording head in which at least a portion thereof disposed adjacent a gap is formed of a soft magnetic material having a saturation magnetic flux density of at least 0.7 T and a reproducing head which is formed of a soft magnetic material having magnetoresistance effect.
Preferably, hexagonal ferromagnetic submicron particles are barium ferrite submicron particles and the magnetic layer has a coercivity squareness ratio (S*) of at least 0.80 as measured in the direction of movement relative to the head.
In a second form of the invention directed to a method for magnetically recording and reproducing signals in and from a magnetic recording medium via a thin film type magnetic head, the magnetic recording medium includes a non-magnetic substrate and a magnetic layer containing a hexagonal ferrite magnetic material. The magnetic layer has a thickness of up to 0.5 .mu.m and a squareness ratio (S.perp.) of at least 0.70 as measured in a direction perpendicular to the recording surface thereof. The magnetic head includes a recording head in which at least a portion thereof disposed adjacent a gap is formed of a soft magnetic material having a saturation magnetic flux density of at least 0.7 T and a reproducing head which is formed of a soft magnetic material having magnetoresistance effect. The magnetic head produces a reproduction output which is subject to zero-crossing detection without passing through a differential circuit.
Preferably, the hexagonal ferrite magnetic material is a barium ferrite magnetic material and the magnetic layer has a coercivity squareness ratio (S.perp.*) of at least 0.90 as measured in a direction perpendicular to the recording surface thereof. Also preferably, the medium is a coated magnetic recording medium having a magnetic layer formed by coating a magnetic coating composition and the magnetic layer has a surface roughness (Ra) of up to 5 nm. Further preferably, the magnetic layer has a coercivity (Hc) of at least 1,100 Oe as measured in the direction of movement relative to the head.
In either form, the head preferably has an inductance of up to 2 .mu.H.